Primary Reaction Channels Research Articles

Ab Initio and Dynamics Study of the O(3P) + NH3 and O(3P) + N2H4 Reactions at Hyperthermal Collision Energies

The reactions between ground-state oxygen atoms (O((3)P)) and the ammonia (NH(3)) and hydrazine (N(2)H(4)) molecules have been studied using electronic-structure and dynamics calculations. Ab initio calculations have been used to characterize the primary reaction channels accessible at hyperthermal energies. These reaction channels are i) hydrogen abstraction, O + NH(3)(N(2)H(4)) --> OH + NH(2)(N(2)H(3)), ii) H-elimination O + NH(3)(N(2)H(4)) --> H + ONH(2)(ON(2)H(3)), and iii) N-N breakage (in the reaction involving hydrazine), O + N(2)H(4) --> ONH(2) + NH(2). Hydrogen abstraction is the lowest-barrier process, followed by N-N breakage and H-elimination. Comparison of our highest-accuracy calculations (CCSD(T)/CBS//MP2/aug-cc-pVDZ) with a variety of lower-cost electronic-structure methods shows that the BHandHLYP method, in combination with the 6-31G* basis set, captures remarkably well the essential features of the potential-energy surface of all of the reaction channels investigated in this work. Using directly the BHandHLYP/6-31G* combination, we have propagated quasiclassical trajectories to characterize the dynamics of the O + NH(3) and O + N(2)H(4) reactions at hyperthermal energies. The trajectory calculations reveal that hydrogen abstraction is the dominant reaction channel, with cross sections between a factor of 2 and an order of magnitude larger than those for the H-elimination and N-N breakage channels. The dynamics calculations also indicate that most of the energy is partitioned into products relative translation but significant vibrational excitation of products is possible as well. Analysis of angular distributions and opacity functions suggests that whereas the hydrogen-abstraction reactions proceed through a mechanism with a substantial component of stripping dynamics, H-elimination and N-N breakage are dominated by rebound dynamics.

Gas-Phase Reactions of Atomic Lanthanide Cations with Ammonia: Room-Temperature Kinetics and Periodicity in Reactivity

Reactions of (14) atomic lanthanide cations (excluding Pm(+)) with ammonia have been surveyed in the gas phase by using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Primary reaction channels were observed corresponding to H(2) elimination with formation of the protonated lanthanum nitride and NH(3) addition. H(2) elimination was seen only in the reactions with La(+), Ce(+), Gd(+), and Tb(+) and occurs with these ions exclusively. NH(3) addition was seen with Pr(+), Nd(+), Sm(+), Eu(+), Dy(+), Ho(+), Er(+), Tm(+), Yb(+), and Lu(+). Higher-order sequential addition of up to five NH(3) molecules was observed with the Ln(+)(NH(3)) and LnNH(+) ions. The reaction efficiency of the primary reactions is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches quite closely the periodic trend in the electron-promotion energy required to achieve a d(1)s(1) or d(2) excited electronic configuration in the lanthanide cation. With La(+), Ce(+), Gd(+), and Tb(+), the electrostatic attraction between the atomic lanthanide cation and ammonia is sufficiently strong to provide enough energy to achieve electron promotion and to overcome any barriers to subsequent N-H bond insertion and H(2) loss, but this is not the case with the other lanthanide cations with which collisional stabilization of the intermediate adduct ion, with or without insertion of Ln(+), predominates.

Reactions of Atomic Cations with Methane: Gas Phase Room-Temperature Kinetics and Periodicities in Reactivity

Reactions of methane have been measured with 59 atomic metal cations at room temperature in helium bath gas at 0.35 Torr using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations were produced at approximately 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K(+) to Se(+), of fifth-row atomic cations from Rb(+) to Te(+) (excluding Tc(+)), of sixth-row atomic cations from Cs(+) to Bi(+), and of the lanthanide cations from La(+) to Lu(+) (excluding Pm(+)). Two primary reaction channels were observed: C-H bond insertion with elimination of H(2), and CH(4) addition. The bimolecular H(2) elimination was observed in the reactions of CH(4) with As(+), Nb(+), and some sixth-row metal cations, i.e., Ta(+), W(+), Os(+), Ir(+), Pt(+); secondary and higher-order H(2) elimination was observed exclusively for Ta(+), W(+), and Ir(+) ions. All other transition-metal cations except Mn(+) and Re(+) were observed to react with CH(4) exclusively by addition, and up to two methane molecules were observed to add sequentially to most transition-metal ions. CH(4) addition was also observed for Ge(+), Se(+), La(+), Ce(+), and Gd(+) ions, while the other main-group and lanthanide cations did not react measurably with methane.

Competitive Activation of CH and CX Bonds in Reactions of Pt+ with CH3X (X=F,Cl): Experiment and Theory

The reactions of Pt(+) with CH(3)X (X=F, Cl) are studied experimentally by employing an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer and theoretically by density functional theory. Dehydrogenation and HX elimination are found to be the primary reaction channels in the remarkably different ratios of 95:5 and 60:40 in the fast reactions of Pt(+) with CH(3)F and CH(3)Cl, respectively. The observed kinetics are consistent with quantum chemistry calculations, which indicate that both channels in the reaction with CH(3)F are exothermic with ground-state Pt(+)((2)D), but that HF elimination is prohibited kinetically because of a transition state that lies above the reactant entrance. The observed HF-elimination channel is attributed to a slow reaction of CH(3)F with excited-state Pt(+)((4)F) for which calculations predict a small barrier. The calculations also show that both the HCl-elimination and dehydrogenation channels observed with CH(3)Cl are thermodynamically and kinetically allowed, although the state-specific product distributions could not be ascertained experimentally. Further CH(3)F addition is observed with the primary products to produce PtCH(2) (+)(CH(3)F)(1,2) and PtCHF(+)(CH(3)F)(1,2). With CH(3)Cl, sequential HCl elimination is observed with PtCH(2) (+) to form PtC(n)H(2) (n) (+) with n=2, 3, which then add CH(3)Cl sequentially to form PtC(2)H(4) (+)(CH(3)Cl)(1-3) and PtC(3)H(6) (+)(CH(3)Cl)(1,2). Also, sequential addition is observed for PtCHCl(+) to form PtCHCl(+)(CH(3)Cl)(1,2).

Water effect on peroxy radical measurement by chemical amplification: Experimental determination and chemical mechanism

The water effect on peroxy radical measurement by chemical amplification was determined experimentally for HO2 and HO2+OH, respectively at room temperature (298±2) K and atmospheric pressure (1×105 Pa). No significant difference in water effect was observed with the type of radicals. A theoretical study of the reaction of HO2·H2O adduct with NO was performed using density functional theory at CCSD(T)/6-311 G(2d, 2p)//B3LYP/6-311 G(2d, 2p) level of theory. It was found that the primary reaction channel for the reaction is HO2·H2O+NO→HNO3+H2O (R4a). On the basis of the theoretical study, the rate constant for (R4a) was calculated using Polyrate Version 8.02 program. The fitted Arrenhnius equation for (R4a) is k = 5.49×107 T 1.03exp(−14798/T) between 200 and 2000 K. A chemical model incorporated with (R4a) was used to simulate the water effect. The water effect curve obtained by the model is in accordance with that of the experiment, suggesting that the water effect is probably caused mainly by (R4a).

Heavy Water Reactions with Atomic Transition-Metal and Main-Group Cations: Gas Phase Room-Temperature Kinetics and Periodicities in Reactivity

Reactions of heavy water, D(2)O, have been measured with 46 atomic metal cations at room temperature in a helium bath gas at 0.35 Torr using an inductively coupled plasma/selected ion flow tube tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and were allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K+ to Se+, of fifth-row atomic cations from Rb+ to Te+ (excluding Tc+), and of sixth-row atomic cations from Cs+ to Bi+. Primary reaction channels were observed leading to O-atom transfer, OD transfer, and D2O addition. O-Atom transfer occurs almost exclusively (>or=90%) in the reactions with most early transition-metal cations (Sc+, Ti+, V+, Y+, Zr+, Nb+, Mo+, Hf+, Ta+, and W+) and to a minor extent (10%) with one main-group cation (As+). OD transfer is observed to occur only with three cations (Sr+, Ba+, and La+). Other cations, including most late transition and main-group cations, were observed to react with D2O exclusively and slowly by D2O addition or not at all. O-Atom transfer proceeds with rate coefficients in the range of 8.1 x 10(-13) (As+) to 9.5 x 10(-10) (Y+) cm3 molecule(-1)(s-1) and with efficiencies below 0.1 and even below 0.01 for the fourth-row atomic cations V+ (0.0032) and As+ (0.0036). These low efficiencies can be understood in terms of the change in spin required to proceed from the reactant to the product potential energy surfaces. Higher order reactions are also measured. The primary products, NbO+, TaO+, MoO+, and WO+, are observed to react further with D(2)O by O-atom transfer, and ZrO+ and HfO+ react further through OD group abstraction. Up to five D(2)O molecules were observed to add sequentially to selected M+ and MO+ as well as MO2+ cations and four to MO(2)D+. Equilibrium measurements for sequential D(2)O addition to M+ are also reported. The periodic variation in the efficiency (k/k(c)) of the first addition of D(2)O appears to be similar to the periodic variation in the standard free energy (DeltaG degrees) of hydration.

Gas-Phase Reactions of Atomic Lanthanide Cations with CO2 and CS2: Room-Temperature Kinetics and Periodicities in Reactivity

Gas-phase reactions of atomic lanthanide cations (excluding Pm+) have been surveyed systematically with CO2 and CS2 using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Observations are reported for reactions with La+, Ce+, Pr+, Nd+, Sm+, Eu+, Gd+, Tb+, Dy+, Ho+, Er+, Tm+, Yb+, and Lu+ at room temperature (295 +/- 2 K) in helium at a total pressure of 0.35 +/- 0.02 Torr. The observed primary reaction channels correspond to X-atom transfer (X = O, S) and CX2 addition. X-atom transfer is the predominant reaction channel with La+, Ce+, Pr+, Nd+, Gd+, Tb+, and Lu+, and CX2 addition occurs with the other lanthanide cations. Competition between these two channels is seen only in the reactions of CS2 with Nd+ and Lu+. Rate coefficient measurements indicate a periodicity in the reaction efficiencies of the early and late lanthanides. With CO2 the observed trends in reactivity across the row and with exothermicity follow trends in the energy required to achieve two unpaired non-f valence electrons by electron promotion within the Ln+ cation that suggest the presence of a kinetic barrier, in a manner much like those observed previously for reactions with isoelectronic N2O. In contrast, no such barrier is evident for S-atom transfer from the valence isolectronic CS2 molecule which proceeds at unit efficiency, and this is attributed to the much higher polarizability of CS2 compared to CO2 and N2O. Up to five CX2 molecules were observed to add sequentially to selected Ln+ and LnX+ cations.

Reactions of Methyl Fluoride with Atomic Transition-Metal and Main-Group Cations: Gas-Phase Room-Temperature Kinetics and Periodicities in Reactivity

Reactions of CH(3)F have been surveyed systematically at room temperature with 46 different atomic cations using an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of fourth-period atomic ions from K(+) to Se(+), of fifth-period atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of sixth-period atomic ions from Cs(+) to Bi(+). Primary reaction channels were observed corresponding to F atom transfer, CH(3)F addition, HF elimination, and H(2) elimination. The early-transition-metal cations exhibit a much more active chemistry than the late-transition-metal cations, and there are periodic features in the chemical activity and reaction efficiency that maximize with Ti(+), As(+), Y(+), Hf(+), and Pt(+). F atom transfer appears to be thermodynamically controlled, although a periodic variation in efficiency is observed within the early-transition-metal cations which maximizes with Ti(+), Y(+), and Hf(+). Addition of CH(3)F was observed exclusively (>99%) with the late-fourth-period cations from Mn(+) to Ga(+), the fifth-period cations from Ru(+) to Te(+), and the sixth-period cations from Hg(+) to Bi(+) as well as Re(+). Periodic trends are observed in the effective bimolecular rate coefficient for CH(3)F addition, and these are consistent with expected trends in the electrostatic binding energies of the adduct ions and measured trends in the standard free energy of addition. HF elimination is the major reaction channel with As(+), while dehydrogenation dominates the reactions of W(+), Os(+), Ir(+), and Pt(+). Sequential F atom transfer is observed with the early-transition-metal cations, with the number of F atoms transferred increasing across the periodic table from two to four, maximizing at four for the group 5 cations Nb(+)(d(4)) and Ta(+)(d(3)s(1)), and stopping at two with V(+)(d(4)). Sequential CH(3)F addition was observed with many atomic cations and all of the metal mono- and multifluoride cations that were formed.

Gas‐Phase Reactions of Atomic Lanthanide Cations with D2O: Room‐Temperature Kinetics and Periodicity in Reactivity

Reactions of atomic lanthanide cations (excluding Pm+) with D2O have been surveyed in the gas phase using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35+/-0.01 Torr and 295+/-2 K. Primary reaction channels were observed corresponding to O-atom transfer, OD transfer and D2O addition. O-atom transfer is the predominant reaction channel and occurs exclusively with Ce+, Nd+, Sm+, Gd+, Tb+ and Lu+. OD transfer is observed exclusively with Yb+, and competes with O-atom transfer in the reactions with La+ and Pr+. Slow D2O addition is observed with early lanthanide cation Eu+ and the late lanthanide cations Dy+, Ho+, Er+ and Tm+. Higher-order sequential D2O addition of up to five D2O molecules is observed with LnO+ and LnOD+. A delay of more than 50 kcal mol(-1) is observed in the onset of efficient exothermic O-atom transfer, which suggests the presence of kinetic barriers of perhaps this magnitude in the exothermic O-atom transfer reactions of Dy+, Ho+, Er) and Tm+ with D2O. The reaction efficiency for O-atom transfer is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches the periodic trend in the electron-promotion energy required to achieve a d1s1 or d2 excited electronic configuration in the lanthanide cation, and also the periodic trends across the lanthanide row reported previously for several alcohols and phenol. An Arrhenius-like correlation is also observed for the dependence of D2O reactivity on promotion energy for early lanthanide cations, and exhibits a characteristic temperature of 2600 K.

Energy Flow and Fragmentation Dynamics of N,N-Dimethylisopropylamine

The energy flow and fragmentation dynamics of N,N-dimethylisopropylamine (DMIPA) upon excitation to the 3p Rydberg states has been investigated with use of time-resolved photoelectron and mass spectrometry. The 3p states are short-lived, with a lifetime of 701 +/- 45 fs. From the time dependence of the photoelectron spectra, we infer that the primary reaction channel leads to the 3s level, which itself decays to the ground state with a decay time of 87.9 +/- 10.2 ps. The mass spectrum reveals fragmentation with cleavage at the alpha C-C bond, indicating that the energy deposited in vibrations during the internal conversion from 3p to 3s exceeds the bond energy. A thorough examination of the binding energies and temporal dynamics of the Rydberg states, as well as a comparison to the related fragmentation of N,N-dimethyl-2-butanamine (DM2BA), suggests that the fragments are formed on the ion surfaces, i.e., after ionization and on a time scale much slower than the fluorescence decay from 3s to the ground state.

Carbon Disulfide Reactions with Atomic Transition-Metal and Main-Group Cations: Gas-Phase Room-Temperature Kinetics and Periodicities in Reactivity

The reactions of 46 atomic-metal cations with CS2 have been investigated at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and products were measured for the reactions of fourth-period atomic ions from K+ to Se+, of fifth-period atomic ions from Rb+ to Te+ (excluding Tc+), and of sixth-period atomic ions from Cs+ to Bi+. Primary reaction channels were observed leading to S-atom transfer, CS2 addition and, with Hg+, electron transfer. S-atom transfer appears to be thermodynamically controlled and occurs exclusively, and with unit efficiency, in the reactions with most early transition-metal cations (Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+) and with several main-group cations (As+, Sb+) and less efficiently with Se+, Re+ and Os+. Other ions, including most late transition and main-group metal cations, react with CS2 with measurable rates mostly through CS2 addition or not at all (K+, Rb+, Cs+). Traces of excited states (< 10%) were seen from an inspection of the observed product ions to be involved in the reactions with Mo+, Te+, Ba+ and Au+ and possibly Pt+ and Ir+. The primary products YS+, ZrS+, NbS+, HfS+, TaS+, WS+, ReS+ and OsS+ react further by S-atom transfer to form MS2(+), and TaS2(+) reacts further to form TaS3(+). CS2 addition occurs with the cations MCS2(+), MS+, MS2(+), CS2(+), and TaS3(+) to form M+(CS2)(n) (n < or = 4), MS+(CS2)(n) (n < or = 4), MS2(+)(CS2)(n) (n < or = 3), (CS2)2(+) and TaS3(+)(CS2). Up to four CS2 molecules add sequentially to bare metal cations and monosulfide cations, and three to disulfide cations. Equilibrium constant measurements are reported that provide some insight into the standard free energy change for CS2 ligation. Periodic variations in deltaG degrees are as expected from the variation in electrostatic attraction, which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CS2.

Ab initio investigations of the radical-radical reaction of O(P3)+C3H3

We present ab initio calculations of the reaction of ground-state atomic oxygen [O(P3)] with a propargyl (C3H3) radical based on the application of the density-functional method and the complete basis-set model. It has been predicted that the barrierless addition of O(P3) to C3H3 on the lowest doublet potential-energy surface produces several energy-rich intermediates, which undergo subsequent isomerization and decomposition steps to generate various exothermic reaction products: C2H3+CO, C3H2O+H, C3H2+OH, C2H2+CHO, C2H2O+CH, C2HO+CH2, and CH2O+C2H. The respective reaction pathways are examined extensively with the aid of statistical Rice-Ramsperger-Kassel-Marcus calculations, suggesting that the primary reaction channel is the formation of propynal (CHCCHO)+H. For the minor C3H2+OH channel, which has been reported in recent gas-phase crossed-beam experiments [H. Lee et al., J. Chem. Phys. 119, 9337 (2003); 120, 2215 (2004)], a comparison on the basis of prior statistical calculations is made with the nascent rotational state distributions of the OH products to elucidate the mechanistic and dynamic characteristics at the molecular level.

Crossed molecular beam reactive scattering: from simple triatomic to multichannel polyatomic reactions

In our laboratory a recent series of experiments by means of the crossed molecular beam (CMB) scattering technique with mass-spectrometric detection and time-of-flight analysis has been instrumental in fostering progress in the understanding of the dynamics of both simple triatomic insertion reactions and complex polyatomic addition–elimination reactions exhibiting competing channels. In the first part of this review we survey the advances made in the comprehension of the dynamics of the insertion reactions involving excited carbon, nitrogen and oxygen atoms – C(1D), N(2D), O(1D) – with H2(D2), as made possible by synergistic comparisons of experimental reactive differential cross-sections with the results of exact quantum, quasiclassical trajectory and statistical calculations on reliable ab initio potential energy surfaces. Related experimental and theoretical work from other laboratories is noted throughout. In the second part, we review the progress made in the understanding of the dynamics of polyatomic multichannel reactions, such as those of ground state oxygen and carbon atoms, O(3P) and C(3P), with the simplest alkyne, acetylene, and alkene, ethylene, as made possible by the gained capability of identifying virtually all primary reaction channels, characterising their dynamics, and determining their branching ratios. Such a capability is based on an improved crossed molecular beam instrument which features product detection by low-energy electron soft-ionisation for increased sensitivity and universal detection power, and variable beam crossing angle for a larger collision energy range and increased angular and velocity resolution. The scattering results are rationalised with the assistance of theoretical information from other laboratories on the stationary points and product energetics of the relevant ab initio potential energy surfaces. These detailed studies on polyatomic multichannel reactions provide an important bridge between crossed beam dynamics and thermal kinetics research. Contents PAGE 1. Introduction 110 2. Experimental: improved crossed molecular beams apparatus 113 2.1. Soft electron-ionisation detection 114 2.2. Product angular and velocity distributions 116 2.3. Crossed molecular beam experiments with variable beam crossing angle 118 2.4. Supersonic sources of radical beams 120 3. Triatomic insertion reactions 120 3.1. Reaction C(1D) + H2 122 3.2. Reaction N(2D) + H2 126 3.3. Reaction O(1D) + H2 129 4. Polyatomic multichannel reactions 133 4.1. Reaction O(3P) + C2H2 134 4.1.1. Product angular and TOF distributions 135 4.1.2. Determination of branching ratios 138 4.1.3. Determination of product ionisation energies 138 4.2. Reaction C(3P) + C2H2 138 4.2.1. H and H2 elimination channels, and branching ratios 140 4.2.2. Ionisation energy of c/l-C3H radicals 145 4.3. Reaction O(3P) + C2H4 145 4.3.1. Observation of all product channels and branching ratios 146 4.4. Reaction C(3P) + C2H4 151 4.4.1. H-elimination channels 151 4.4.2. C–C bond fission channels 154 5. Summary and outlook 154 Acknowledgements 157 References 158

A PM3-SRP + Analytic Function Potential Energy Surface Model for O(3P) Reactions with Alkanes. Application to O(3P) + Ethane

The PM3 semiempirical electronic structure theory is reparametrized with specific reaction parameters (SRPs) to develop a potential energy surface (PES) for O(3P) processing of alkanes. The results of high-level ab initio calculations for the O(3P) + C2H6 primary reactions, yielding OH + C2H5, C2H5O + H, and CH3O + CH3, 11 ensuing secondary and unimolecular dissociation reactions involving products of these primary reactions, and additional reactions were used to develop two PM3-SRP models for the PES. The ab initio results used for this fitting were taken from previous multiconfiguration calculations and additional PMP2/cc-pVTZ calculations reported here. Even though these two PM3-SRP models are unable to quantitatively represent the many reactions that occur in high-energy collisions of O(3P) with alkanes, they are vast improvements over the PES of PM3 theory. These models are used in direct dynamics classical trajectory simulations of the O(3P) + C2H6 reaction at a 5 eV collision energy. The results of the simulations show that the products of the three primary reactions are highly excited and are able to undergo a large number of ensuing secondary and unimolecular dissociation reactions, and long-time trajectory integrations are required to study these many product channels. The large internal excitations of the primary reactions' products agree with results of a previous MSINDO direct dynamics trajectory study. Reaction cross sections calculated for the primary reaction channels are also in good agreement with the MSINDO results. Velocity scattering angles, calculated for products of the secondary and unimolecular dissociation channels, provide detailed information concerning the molecular dynamics of these products. They are formed directly and also via long-lived intermediates.

Gas-Phase Oxidation and Nitration of First-, Second-, and Third-Row Atomic Cations in Reactions with Nitrous Oxide: Periodicities in Reactivity

Room-temperature reactions of 46 different atomic cations with N2O have been surveyed systematically using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations are produced at about 5500 K in the ICP source and are allowed to decay radiatively and thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic cations from K+ to Se+, of second-row atomic cations from Rb+ to Te+ (excluding Tc+), and of third-row atomic cations from Cs+ to Bi+. Primary reaction channels were observed corresponding to O-atom transfer, N-atom transfer, and N2O addition. Periodicities were observed in overall reaction efficiency, and these are scrutinized in terms of overall exothermicity, the presence of an activation barrier in the reaction coordinate, and the overall conservation of spin. N-atom transfer was observed to compete with O-atom transfer in the reactions of N2O with La+ (4%), Ti+ (22%), Zr+ (45%), Nb+ (35%), Ta+ (40%), and Os+ (60%). Up to three N2O molecules were observed to add sequentially to selected atomic cations as well as several monoxide and dioxide cations. A second O-atom transfer was observed with the group 4, 5, and 6 transition-metal ions (except Mo+) as well as the third-row cations Re+, Os+, Ir+, and Pt+. The atomic ions W+, Os+, and Ir+ formed trioxides in sequential O-atom transfer reactions, and Os+ even formed the tetroxide OsO4+. Multicollision-induced dissociation studies with Ar buffer gas indicated thermodynamically controlled dissociation of TiO2+, ZrO2+, HfO2+, VO2+, NbO2+, TaO2+, and WO2+ by the consecutive detachment of O-atoms while CrO2+, ReO2+, and PtO2+ decomposed primarily by loss of O2 molecules.

Surface-induced dissociation of ions produced by matrix-assisted laser desorption/ionization in a fourier transform ion cyclotron resonance mass spectrometer.

Intermediate pressure matrix-assisted laser desorption/ionization (MALDI) source was constructed and interfaced with a 6-T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for surface-induced dissociation (SID) studies. First MALDI-SID results in FT-ICR are presented, demonstrating unique advantages of SID over conventional FT-ICR MS ion activation techniques for structural characterization of singly protonated peptide ions. Specifically, we demonstrate that SID on a diamond surface results in a significantly better sequence coverage for singly protonated peptides than SORI-CID. A combination of two effects contributes to the improved sequence coverage: shattering of peptide ions on surfaces opens up a variety of dissociation channels at collision energies above 40 eV, and second, wide internal energy distribution deposited by collision with a stiff diamond surface provides an efficient mixing between the primary reaction channels that are dominant at low internal energies and extensive fragmentation at high internal excitation that results from shattering. Activation of MALDI-generated ions by collisions with surfaces in FT-ICR MS is a new powerful method for characterization and identification of biomolecules

Gas-Phase Reactions of Transition-Metal Ions with Molecular Oxygen: Room-Temperature Kinetics and Periodicities in Reactivity

An Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer has been employed in a systematic survey of room- temperature reactions of O2 with 29 transition-metal ions. The atomic ions are produced at ca. 5500 K in an ICP source and are allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction. Rate coefficients were measured for the reactions of first-row atomic ions from Sc+ to Zn+, of second-row atomic ions from Y+ to Cd+ (excluding Tc+) and of third-row atomic ions from La+ to Hg+. Both O-atom abstraction and O2 addition were observed as primary reaction channels. Periodicities in reactivity are identified and are compared with periodicities in the O-atom affinity of the atomic ion. O-atom abstraction was observed to be efficient when exothermic and inefficient when endothermic. NbO+, MoO+, TaO+, and WO+ reacted further in a second O-atom transfer reaction with O2. ReO2+ produced by direct addition of O2 to Re+ reacted further by both O-atom abstraction and O2 addition. Sequential O2 addition was observed to produce the higher oxides: MO3+ (M = Ti, V, Y, Zr, Hf, Os), MO4+ (M = Cr, Fe, Co, Ni, Cu, Nb, Mo, Ru, Rh, Hf, Ta, W, Re, Os, Ir, Pt, Au), MO5+ (M = V, Re, Os), MO6+ (M = Nb, W, Re) and MO7+ (M = V). Novel termolecular reactions second-order in oxygen were identified for Os+ and ZrO+ at high oxygen concentrations to produce O2+ and OsO+ from Os+ and a neutral metal oxide from Zr+.

Charge effects for differentiation of oligodeoxynucleotide isomers containing 8-oxo-dG residues

Dissociation reactions of a series of multiply charged oligodeoxynucleotide (ODN) 12-mer anions were studied using an ion trap mass spectrometer. These mixed nucleobase 12-mers fragment first by loss of a neutral nucleobase (A, G, C, and/or 5-methyl-cytosine) followed by cleavage at 3′ CO bond of the sugar from which the base is lost to produce the complementary sequence ions, i.e., [a – B] and w type of ions. No detectable loss of 8-oxo-guanine and/or thymine from these 12-mers is observed under gentle collision conditions in the ion trap. The primary loss of a nucleobase and the subsequent backbone cleavage to generate sequence ions strongly depend on the charge state of the parent molecular ion. For low charge states (2- and 3-), product ions due to the loss of a neutral guanine base and related sequence ions are dominant in the tandem mass spectra. However, preferential loss of a neutral adenine becomes the primary reaction channel from the 5- charge state of the molecular ion. Such charge state dependent fragmentation behavior was utilized to determine the site of 8-oxo-dG residue in a series of structural isomers. The position of 8-oxo-dG residue can be simply determined from the fragmentation pattern of 3- charge state, but not of 5- charge state. It is suggested that in addition to specific modification that affects the N-glycosidic bond strength, total charge content of an ODN is an important factor for determining the differential fragmentation behavior.

C–Cl bond fission, HCl elimination, and secondary radical decomposition in the 193 nm photodissociation of allyl chloride

The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C–Cl bond fission channel producing unstable allyl radicals, an excited state C–Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C–Cl] : [fast C–Cl] : [slow C–Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167: 0.458 (where fast refers to the high recoil kinetic energy channels). The high internal energy allyl radicals formed in the slow C–Cl fission pathway of allyl chloride further dissociate/isomerize, as do the unstable mass 40 products formed in the HCl elimination pathways, and these products are investigated. Photoionization efficiency (PIE) curves of the HCl product suggest that a three-centered elimination mechanism contributes significantly to an observed HCl elimination reaction.

Reactions of carbon dioxide radical anion with substituted benzenes

AbstractWe report mechanistic studies on the reactivity of CO2 .− radicals towards substituted benzenes, XPh, with X = NO2, COOH, COH, CH3, H, Cl and HO. Vacuum ultraviolet (VUV, 172 nm) irradiation of aqueous solutions containing formate was used as a method of CO2 .− radical generation for the study of the reaction products, which were determined by gas chromatography–mass spectrometry and high‐performance liquid chromatography. The nature of some of the reaction intermediates was studied by laser flash photolysis of aqueous solutions of peroxodisulfate in the presence of formate. The observed products and intermediates of reaction are discussed and rationalized by a dual reactivity of the CO2 .− radical anion with substituted benzenes: a one‐electron reduction and radical addition to the aromatic ring. For substituted benzenes with X = NO2, COOH and COH, the observed products support an electron transfer from CO2 .− to the substrates as the primary reaction channel, while the reactions with toluene follow the channel of radical addition. Benzene and chlorobenzene react with CO2 .− by both routes, electron transfer and radical addition. Our experimental results indicate that H abstraction by the CO2 .− radical is negligible under the chosen experimental conditions. Copyright © 2001 John Wiley &amp; Sons, Ltd.